As the storage capacity of magnetic recording disks is increased by reducing track width and transducing gaps, the heads must fly closer and closer to the recording surface in order to maintain adequate signal strength. If the head contacts the recording surface while the disk is moving, the resultant "head crash" can wipe out previously recorded data, damage the recording surface, and/or the slider on which the head is mounted.
The effect of head/media spacing on the amplitude of magnetic readback signals is described by R. L. Wallace, Jr. in "The Reproduction of Magnetically Recorded Signals", The Bell System Technical Journal, Vol. 30, October 1951, pp. 1145-1173. This publication includes the "Wallace equation" which expresses the dependence of readback voltage on various recording parameters including head/disk spacing.
U.S. Pat. No. 3,686,682 describes a method and apparatus wherein a test signal consisting of pairs of positive and negative-going pulses is recorded on a selected track of a recording disk. The time interval between the peaks of the pulses of each pair is measured. The number of pulse pairs whose spacing varies a predetermined amount relative to the normal pulse spacing, and also the number of pulses whose peak amplitudes varies from an average level by some predetermined amount, are measured to provide a qualitative determination of the performance characteristics of the system.
U.S. Pat. No. 4,872,071 describes a method and apparatus for detecting abnormal operation of a magnetic disk file by reading data from a predetermined area of each track in order to generate a readback signal. The amplitude of these readback signals from each track is compared with the amplitude of that read back from other tracks. If the comparison indicates a predetermined degree of variation, an alarm is given so that corrective action can be taken to avoid an impending head crash.
U.S. Pat. No. 4,841,389 describes a self-diagnostic method and apparatus that dynamically and periodically indicates the likelihood of an impending head crash based upon contemporary readback signal inputs and upon signal overwrite efficiency.
U.S. Pat. No. 4,777,544 describes another method and apparatus of incidental interest which uses the above-mentioned Wallace equation to measure head/disk clearance in situ.
None of these prior art references or any other prior art of which applicants are aware teaches measurement of transducer-to-recording medium clearance or determining transducer instability by sensing the pulse width of a readback signal at a predetermined amplitude level which is a preselected percentage (e.g., 50%) of its base-to-peak amplitude to provide a voltage corresponding to said pulse width at said level, and comparing said voltage to a predetermined prestored reference value. Applicants have found that a substantially linear relationship exists between the pulse width and the transducer-to-medium clearance. For a particular transducer, the slope of a plot of pulse width vs. flyheight/clearance varies according to the amplitude level at which the pulse width is sensed.
There is a need for a method and circuitry that can be applied to existing magnetic recording channel circuitry with minimal expense and without requiring special transducers or read-write circuits (a) to study, from the instantaneous readback signal for diagnostic purposes, the dynamics of the air bearing generated as a transducer flies above the moving storage medium, (b) to indicate magnetic instability of the transducer, and/or (c) to dynamically and periodically indicate the likelihood of an impending crash of a magnetic transducer with a magnetic recording medium.